The present invention relates to a process for the electrochemical oxidation of a semiconductor substrate that has recesses formed in a silicon surface region. The invention relates, in particular, to the formation of oxide in trenches, for example, capacitor trenches, or mesopores which are formed in a silicon substrate.
The present invention can be used particularly advantageously for fabricating DRAM (dynamic random access memory) memory cells. Memory cells of this type, which are produced almost exclusively as single-transistor memory cells, generally include a read transistor and a storage capacitor. The information is stored in the storage capacitor in the form of an electric charge that represents a logic 0 or a logic 1. Actuating the read transistor via a word line allows this information to be read via a bit line. The storage capacitor must have a minimum capacitance for reliably storing the charge, and to make it possible to differentiate the information item which has been read. The lower limit for the capacitance of the storage capacitor is currently considered to be 25 fF.
Since the storage density increases from memory generation to memory generation, the surface area required by the single-transistor memory cell must be reduced from generation to generation. At the same time, the minimum capacitance of the storage capacitor has to be retained.
Up to the 1 Mbit generation, both the read transistor and the storage capacitor were produced as planar components. Beyond the 4 Mbit memory generation, the area taken up by the memory cell was reduced further by using a three-dimensional arrangement of the read transistor and the storage capacitor. One possibility is for the storage capacitor to be produced in a trench. In this case, a diffusion region that adjoins the wall of the trench and a doped polysilicon filling arranged in the trench act as electrodes for the storage capacitor. Therefore, the electrodes of the storage capacitor are arranged along the surface of the trench. In this way, the effective surface area of the storage capacitor, on which the capacitance is dependent, is increased with respect to the space taken up by the storage capacitor on the surface of the substrate, which corresponds to the cross section of the trench. By reducing the cross section of the trench, it is possible to further increase the packing density. However, for technological reasons there are limits on the extent to which the depth of the trench can be increased.
However, the effective surface area of the storage capacitor and therefore the capacitance of the capacitor can be increased by measures which increase the surface area, such as for example, widening the capacitor trench in the lower region, etching mesopores or by the HSG (hemi-spherical grain) process (roughening of the silicon surface), without increasing the space taken up in the horizontal plane as a result.
As will be explained below, the present invention can be applied particularly advantageously to widening existing recesses. Furthermore, an important application area is the formation of oxide on special geometric structures.
Electrically insulating layers of oxides or nitrides play a very important role in the fabrication of DRAM memory cells. While in earlier DRAM generations, these layers were generally used in planar form and were produced by deposition and/or heat treatment and were then patterned. Since the introduction of the capacitor trenches, increased emphasis has been placed on integrating dielectrics in the form of cylinder jackets (for example as insulation collars for disconnecting a parasitic transistor) or in the form of reagent glasses (for example the node capacitor dielectric).
Hitherto, oxide layers have generally been formed by thermal oxidation under dry or wet conditions (furnace processes) or by deposition. In these processes, the thickness of the oxide layers can be controlled very successfully by setting the reaction time, for example.
In particular in order to widen, for example, capacitor trenches or mesopores, sacrificial oxide layers are used and are subsequently removed again. In this case or also in the case of oxidation of thin active silicon layers of SOI substrates, there may in particular be a need for the silicon material which is present, for example, below the sacrificial oxide layer to be converted into oxide apart from a defined residual thickness. In the processes described above, this can only be achieved by complex process control.
U.S. Pat. No. 6,143,627 discloses a process for the electrochemical oxidation of silicon. In this process, certain parts of a silicon substrate are covered by a nonconductive layer and the silicon substrate is brought into contact with an electrolyte. A voltage is applied between a cathode in the electrolyte and the silicon substrate, so that a reaction between silicon and the electrolyte to form silicon dioxide takes place at the uncovered surface regions of the silicon substrate. A different layer thickness is achieved depending on the applied voltage and the reaction time. According to the invention described in this patent, there is no limiting of the layer growth, i.e. in the diagram shown in FIG. 5 of the patent, the layer thickness grows in strictly monotonous fashion and there is no saturation. Furthermore, no recesses whatsoever are formed in the silicon surface region.
It is accordingly an object of the invention to provide a process for the electrochemical oxidation of a semiconductor substrate with which a predetermined oxide thickness is produced in recesses in a silicon surface region or with which a minimum silicon layer thickness remains between two adjacent recesses.
With the foregoing and other objects in view there is provided, in accordance with the invention, a process for an electrochemical oxidation of a semiconductor substrate, which includes steps of: providing a semiconductor substrate having at least one surface region formed with recesses being spaced apart from one another by a spacing, said surface region having silicon and a predetermined doping; providing an electrolyte in contact with the surface region; applying a voltage between the semiconductor substrate and a cathode configured in the electrolyte to bring about a reaction between the surface region and the electrolyte in order to form silicon oxide; determining that the electrochemical oxidation has ended based on a self-limiting effect; and interrupting the voltage between the semiconductor substrate and the cathode.
In accordance with an added feature of the invention, the self-limiting effect is brought about as a result of reaching a predetermined oxide thickness when the voltage between the semiconductor substrate and the electrolyte is a given voltage and when the electrolyte has a given composition.
In accordance with an additional feature of the invention, the self-limiting effect is brought about as a result of a layer thickness between two adjacent ones of the recesses falling below a minimum residual silicon layer thickness when the voltage between the semiconductor substrate and the electrolyte is a given voltage and when the electrolyte has a given composition.
In accordance with another feature of the invention, the process includes: setting the voltage and a composition of the electrolyte such that a following relationship applies:
dxe2x89xa72*(an extent of the space charge region+an oxide thickness that will be produced);
where d is a layer thickness of the silicon that is between two adjacent ones of the recesses. The extent of the space charge region is dependent on a level of an effective voltage acting over the space charge region, and the effective voltage is dependent on the voltage that is applied and a composition of the electrolyte.
In accordance with a further feature of the invention, the electrolyte includes an agent for etching silicon oxide.
In accordance with a further added feature of the invention, the process is used to oxidize faces of capacitor trenches.
In accordance with yet an added feature of the invention, the process is used to oxidize surfaces of mesopores.
In accordance with yet an additional feature of the invention, the process is used to widen diameters of trenches to form trench capacitors.
In accordance with yet another feature of the invention, the process is used to widen diameters of mesopores.
As shown in FIG. 2, recesses 3 are arranged in the silicon region 2, and the silicon region 2 is brought into contact with an electrolyte 5. A voltage is applied between the semiconductor substrate 1, which acts as an anode, and a cathode 6 in the electrolyte 5. The contact between the electrolyte 5 and the silicon region 2 may be thought of as a Schottky contact. Accordingly, a space charge region 4, the extent of which is indicated by dashed lines, is formed at the interface in the silicon region 2. As is generally known, the width of the space charge region 4 in a Schottky contact is dependent, inter alia, on the effective voltage acting at the silicon surface and on the doping of the silicon region.
At the beginning of the oxidation, the space charge regions 4 of adjacent recesses 3 are not yet in contact with one another. There is a field across the space charge region, through which oxygen ions and OHxe2x88x92 groups diffuse from the electrolyte into the silicon surface region, where they convert the silicon into SiOx.
This diffusion and therefore the oxide formation take place until the electric field is at the silicon surface region. In other words, as soon as the electric field is constricted as a result of the space charge regions 4 of adjacent recesses coming into contact with one another, or the oxide layer produced is so thick that all of the voltage that is present drops off over the oxide layer, the oxide formation comes to a standstill.
Therefore, self-limited oxide formation takes place. The end of which is reached, as a function of the process parameters, such as, the doping of the silicon region, the applied voltage and the composition of the electrolyte used, as soon as either a predetermined maximum layer thickness of the formed oxide or a predetermined minimum residual silicon layer thickness between two adjacent recesses is reached.
The self-limiting effect can occur in particular because the oxide formation takes place in the recesses that are present in the silicon surface region.
This is explained in more detail with reference to FIGS. 3 to 5.
FIG. 3 shows the arrangement of, for example, capacitor trenches 3 before the oxide formation, while FIG. 4 shows the capacitor trenches 3 after the oxide formation. The extent of the space charge regions 4 is once again indicated by dashed lines. The upper part of FIGS. 3 and 4 in each case show a plan view of a cross section parallel to the substrate surface, while the lower part of FIGS. 3 and 4 in each case show a cross section perpendicular to the substrate surface.
As shown in FIG. 4, for the set parameters of the oxide formation, the further formation of oxide is prevented as a result of all of the active voltage dropping off over the thickness of the formed oxide layer that has already been reached, so that the oxygen ions and OHxe2x88x92groups no longer diffuse into the silicon surface region. As a result, as shown in the upper part of FIG. 4, a concentric silicon oxide layer is formed in the recess 3.
Depending on the voltage applied, the oxide layer may be a few nm (nanometers) to over 10 nm thick before the oxide growth stops.
FIG. 5 shows the arrangement of, for example, capacitor trenches after the oxide formation. In this case, the further growth of oxide is prevented as a result of the space charge regions 4 of adjacent trenches 3 coming into contact with one another. Once again, the extent of the space charge regions 4 is indicated by dashed lines. The upper-part of FIG. 5 shows a plan view of a cross section parallel to the substrate surface, while the lower part of FIG. 5 shows a cross section perpendicular to the substrate surface.
In the arrangement shown in FIG. 5, the further growth of oxide is prevented as a result of the space charge regions 4 of adjacent trenches 3 coming into contact with one another.
More precisely, as shown in the lower part of FIG. 5, when a corresponding voltage is applied, the silicon rib between adjacent trenches is formed completely as a space charge region 4, so that at this region there is a very high resistance and therefore there is no longer any field acting at the trench surface through which oxygen ions and OHxe2x88x92 groups could diffuse from the electrolyte into the silicon surface region, where they could convert the silicon into SiOx.
In this case, the thickness of the silicon oxide layer, which has already formed, is much too small in relation to the applied voltage for it to be of importance with regard to limiting the oxide growth. Limiting the oxide growth is achieved only as a result of the space charge regions growing together.
Taking account of the rule that a width of the space charge region of 100 nm corresponds to a resistivity of the silicon substrate of 10 mxcexa9cm, this resistivity being dependent on the doping of the silicon substrate, the residual silicon layer thickness between adjacent recesses can be estimated for a given doping.
As can be seen from the upper part of FIG. 5, the oxide layer adopts a different form from that shown in FIG. 4.
If the further growth of oxide is stopped as a result of the space charge regions of adjacent recesses coming into contact with one another, geometry-dependent influences, such as for example, the arrangement of the pores in adjacent trenches and therefore a premature stop to growth in the directions in which the trenches are narrowest, can also lead to the formation of direction-dependent oxide layers. In particular, by way of example, in the arrangement shown in FIG. 5, only a very thin silicon oxide layer grows on the tips of the silicon ribs between adjacent trenches, since on account of the space charge regions which are forming, depending on the voltage applied, the electric field is largely constricted in the uppermost region of the rib.
Which of the mechanisms that have been described is actually active in limiting the oxide growth is dependent in particular on the doping of the silicon substrate, the size of the recesses formed in the silicon surface region, and the voltage which effectively acts on the substrate surface.
The end point of the electrochemical oxidation can be detected, for example, as a result of no further current flowing between the anode and the cathode. Furthermore, end point detection by measuring current has the advantage that it can easily be automated. The current is monitored by a measuring device and is compared with a threshold value. If the current falls below the threshold value, this is interpreted as indicating the end point of the oxidation. However, other end point detection processes are also conceivable, for example, by monitoring the composition of the electrolyte. As soon as the electrochemical oxidation has ended, the voltage between semiconductor substrate and cathode is interrupted.
According to the present invention, the thickness of the silicon oxide layer that will be produced or the thickness of the residual silicon layer between adjacent recesses can be controlled substantially by two parameters.
One parameter is the applied voltage. If the further growth of oxide is limited by the thickness of the oxide layer which has already been deposited, the electric field which is effectively active and causes the oxygen ions and OHxe2x88x92groups to penetrate into the silicon substrate is given first by the thickness of the oxide layer that has been deposited and second by the level of the applied voltage. The higher the applied voltage, the larger the oxide layer which can be deposited.
By contrast, if the further growth of oxide is limited by the fact that adjacent space charge regions come into contact with one another, the width of the space charge region is, as is known from the theory of Schottky contacts, dependent on the root of the applied voltage. What this means is that the higher the applied voltage, the larger the space charge region becomes. Therefore, the higher the applied voltage, the larger the residual silicon layer between adjacent recesses becomes.
The second parameter involved in setting the oxide thickness which is to be deposited and/or the residual silicon layer is the composition of the electrolyte. Since the voltage is applied between the cathode in the electrolyte and the anode in the silicon substrate, the electric field which is effectively active at the substrate surface is dependent in particular on the conductivity and the pH of the electrolyte. An important factor in this context is the proportion of the voltage that drops off over the electrolyte. For example, when an electrode with a relatively high conductivity is used, with the same voltage applied, a thicker layer of oxide can be deposited then when an electrolyte of lower conductivity is used.
The electrolyte may advantageously contain an agent for etching silicon oxide, such as for example, traces of hydrofluoric acid, if the process according to the invention is going to be used to widen recesses. As a result, the silicon oxide is partially dissolved during the oxidation process, so that the silicon is uniformly removed.
According to the present invention, the voltage and the composition of the electrolyte are preferably set in such a manner that the following relationship applies:
dxe2x89xa72*(the extent of the space charge region+the oxide thickness that will be produced).
The extent of the space charge region is dependent on the level of the effective voltage acting over the space charge region and is dependent on the applied voltage and the composition of the electrolyte, and d indicates the wall thickness between two adjacent recesses. If an agent for etching silicon oxide has already been added to the electrolyte, the formula is reduced to:
dxe2x89xa72*(the extent of the space charge region)
According to the present invention, it is possible in particular for dilute acids, such as for example, a 0.01 mol H2SO4 (sulphuric acid) or a 0.01 mol HCl (hydrochloric acid) or also other known electrolytes for the electrochemical oxidation, such as for example, NH4OH (ammonium hydroxide) to be used as a liquid electrolyte.
The electrochemical oxidation takes place in particular in p-doped or n-doped silicon regions, with the standard dopant concentrations. Typical voltages between the anode and cathode are in a range from 0 volts to a few tens of volts.
The present invention can be used particularly advantageously to oxidize the surface of the capacitor trench, for example in order to form a sacrificial oxide layer which during the subsequent process is removed again at a suitable location. The invention can also be used to oxidize the surface of mesopores, for example, in order to widen them during the subsequent etching of the oxide layer.
Furthermore, the present invention can advantageously be used to widen the diameter of a trench in order to form a trench capacitor or to widen the diameter of mesopores, either as described above, during subsequent etching, or during simultaneous etching of the silicon oxide layer.
Overall, the present invention provides the following advantages:
The process of the present invention allows oxide to be formed uniformly with a predetermined thickness, or allows residual silicon layer thickness to be formed on patterned silicon surfaces of any desired geometry.
The fact that the silicon oxide is formed from the silicon that is present in the silicon surface region means that silicon material is xe2x80x9cconsumedxe2x80x9d. Accordingly, the process according to the invention can advantageously be used to widen the diameter of recesses, such as for example, capacitor trenches or mesopores.
If the process according to the invention is used to apply a silicon oxide layer, for example, as a sacrificial layer, the diameter of the recess is thereby reduced to only a limited extent.
The self-limiting of the oxide formation means that it is possible to set the silicon oxide layer thickness formed or the residual silicon layer thickness.
Compared to thermal oxidation, in the case of anodic oxidation, the thermal budget required is reduced considerably.
The oxide layer that is formed has the same thickness irrespective of the crystal orientation, which in the case of thermal oxidation can only be achieved approximately with very hot processes.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a process for the electrochemical oxidation of a semiconductor substrate, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.